Effect of Non-lockdown Social Distancing and Testing-Contact Tracing During a COVID-19 Outbreak in Daegu, South Korea, February to April 2020: A Modeling Study

Abstract

Objective: In Spring 2020, South Korea applied non-lockdown social distancing (avoiding mass gathering and non-essential social engagement, without restricting the movement of people who were not patients or contacts), testing-and-isolation (testing), and tracing-and-quarantine the contacts (contact tracing) to successfully control the first large-scale COVID-19 outbreak outside China. However, the relative contributions of these two interventions remain uncertain.

Methods: We constructed an SEIR model of SARS-CoV-2 transmission (disproportionately through superspreading events) and fit the model to outbreak data in Daegu, South Korea, from February to April 2020. We assessed the effect of non-lockdown social distancing (population-wide control measures) and/or testing-contact tracing (individual-specific control measures), alone or combined, in terms of the basic reproductive number (R0) and the trajectory of the epidemic.

Results: The point estimate for baseline R0 is 3.6 (sensitivity analyses range: 2.3 to 5.6). Combined interventions of non-lockdown social distancing and testing-contact tracing can suppress R0 to less than one and rapidly contain the epidemic, even under the worst scenario with a high baseline R0 of 5.6. In contrast, either intervention alone will fail to suppress R0. Non-lockdown social distancing alone just postpones the peak of the epidemic, while testing-contact tracing alone only flattens the curve but does not contain the outbreak.

Conclusions: To successfully control a large-scale COVID-19 outbreak, both non-lockdown social distancing and testing-contact tracing must be implemented. The two interventions are synergistic.

Figure 1

Model structure S: Susceptible; E: Latent (Infected but not yet infectious); I_SSE: Infectious and symptomatic superspreaders; I_non-SSE: Infectious and symptomatic non-superspreaders; A_SSE: Infectious but asymptomatic superspreaders; A_non-SSE: Infectious but asymptomatic non-superspreaders; H: Hospitalized due to severe/critical COVID-19; R: Recovered; XQ: Quarantine of uninfected contacts; Q_R: Isolation/quarantine of infected patients/contacts who either develop a mild illness or remain asymptomatic. Q_H: Isolation/quarantine of infected patients/contacts who develop severe/critical illnesses.

Figure 2

Simulating the Daegu outbreak Simulations were conducted under a baseline primary reproductive number (R0) of 3.6 (point estimate, based on epidemiological data from Wuhan, China, January 6 to January 20, 2020). Panel A: Black line shows predicted cumulative numbers of infections in Daegu (by date of transmission in the model) with testing (mean duration from illness onset to confirmation before quarantine: 5.3 days) and 50% contact tracing, started on February 20, 2020, as well as 50% non-lockdown social distancing, started on February 29, 2020 (purple arrows, respectively). To smooth the curve, testing-contact tracing and non-lockdown social distancing started from zero and gradually increased to the targeted level over a period of 10 days and 3 days, respectively. The population size of Shincheonji members and their close contacts in Daegu was assumed as 30,000. Blue shadow shows the sensitivity analysis on outbreak size by the uncertainty range of the Shincheonji population (range: 10,000 to 50,000). The Red line shows the number of cumulative confirmed cases by date from the Korean Center for Disease Control (KCDC) statistics. Panel B: Dark red shadow shows the cumulative numbers of superspreading events (SSEs)-associated infection. Light red shadow shows cumulative numbers of infections that were not associated with SSEs.

Figure 3

Reproductive number (R0) under social distancing alone (A) or testing-contact tracing alone (B) If the R0 is more than one (red color), then the disease will continue to spread. On the other hand, if the R0 is less than 1 (green color), the epidemic can be controlled.

Figure 4

Heterogeneity in infectiousness per individual: (A) Effect of social distancing (population-wide interventions); (B) Effect of testing-contact tracing (individual-specific interventions) Heterogeneity in infectiousness per individual is defined by the ratio between the numbers of secondary infections from superspreaders (I_SSE and A_SSE) and that from non-superspreaders (I_Non-SSE and A_Non-SSE).

Figure 5

Reproductive number (R0) under combined interventions Panel A: Under a baseline R0 of 3.6, combined interventions with 50% non-lockdown social distancing and testing-contact tracing (50%) suppress the R0 to less than 1. Panel B: under a baseline R0 of 5.6, combined interventions with 75% non-lockdown social distancing and testing-contact tracing (75%) suppress the R0 to less than 1.

Figure 6

Trajectories of COVID-19 epidemic under different scenarios Red line: natural course of the epidemic, without interventions. Brown line: non-lockdown social distancing (SD) alone. Green line: testing (with a mean time of 5.3 days from illness onset to confirmation) and contact-tracing (T & T) alone. Blue line: Combined intervention (SD + T & T). All interventions were started when the number of new infections reaches 50 per day (marked by black arrow) in a 30,000 population. The color shadows show sensitivity analyses, ranging from -5% to +5% of the interventions. Panel A: Under a baseline R0 of 3.6, effects of SD 50% alone (brown line), T & T 50% alone (green line), and SD 50% + T &T 50% (blue line). Panel B: Under a baseline R0 of 5.6, effects of SD 75% alone (brown line), T & T 75% alone (green line), and SD 75% + T &T 75% (blue line).